N-Polar GaN Deep Recess HEMTs for mm-Wave Power …

N-Polar GaN Deep Recess HEMTs for mm-Wave

Power Amplification

Brian Romanczyk

Mishra Research Group

Department of Electrical and Computer Engineering

University of California Santa Barbara

With support from ONR (Dr. Paul Maki) and DARPA (Dr. Dan Green, Dr. Y.-K. Chen)

October 10, 2019

Outline

UC Santa Barbara ECE Dept. Mishra Group 2

I. Introduction

• mm-Wave Application Space

• Status of Competing Device Technologies

• Demonstrated Advantage of N-Polar GaN

II. The N-Polar GaN Deep Recess HEMT

• Enabling Features of the Device Structure

• Fabrication Process for Self-Aligned Gate

III. Experimental Results: Large Signal Performance

• W-Band Device Performance (94 GHz)

• Ka-Band Device Performance (30 GHz)

IV. Conclusion

Outline


I. Introduction










IV. Conclusion

mm-Wave Applications


79 GHz: Automotive Radar / Collision Avoidance

Atmospheric absorption windows and attenuation peaks useful

for a variety communication and sensing of applications

Frequency (GHz)

10 15 20 25 30 40 60 80 100 150 200 300 4000.001

0.002

0.004

0.01

0.02

0.04

0.1

0.2

0.4

1

2

4

10

20

40

Att

en

ua

tio

n (

dB

/km

)

H2O

O2

H2O H2O

X Ku K Ka V W G

5G

5G

802.11ad

Automotive

Radar Weather

Radar

Radar

Imaging

E-Band

Radio

mm-Wave (30 – 300 GHz)

W-Band(75–110 GHz)

O2

5G Communication

Solid-State Power Amplifiers


Qorvo TGA2594 datasheet

Transistor

Unit Cell

Qorvo TGA2594: 27-31GHz 5W GaN Power Amplifier

Typical mm-Wave PA’s cascade multiple

transistors to provide useful level of gain

and power

This Work: Fabricate only transistor unit

cells to characterize the device

Stage1 2 3

RFin RFout

G D

S

S

This Work

GaN: Proven Material for Solid-State RF Power Generation


Plotted data from commercial product datasheets and select publications

Data compiled with M. Guidry

GaN provides highest output

power from 1 to 100 GHz

Polarization: Large 2DEG Charge without

Doping (High Mobility & Current)

Wide Bandgap:Large Breakdown Voltage

Key Properties of GaN

(Ga- & N-Polar)

Key Device Metrics for Power Amplifiers


Gain

• High Mobility & Velocity

• Electrostatic Control

• Physical Scaling

• Reduced Parasitics

Output Power

• Large Current Swing

• Large Voltage Swing

• Low Dispersion

mm-Wave

HEMT

𝐺 =𝑃𝑅𝐹,𝑜𝑢𝑡𝑃𝑅𝐹,𝑖𝑛 =

2 𝑉𝐷𝐷 − 𝑉𝑘𝑛𝑒𝑒 𝐼𝑘𝑛𝑒𝑒8

𝑃𝑜𝑢𝑡 =𝑉𝑠𝑤𝑖𝑛𝑔 𝐼𝑠𝑤𝑖𝑛𝑔

8

𝑃𝐴𝐸 =𝑃𝑅𝐹,𝑜𝑢𝑡 − 𝑃𝑅𝐹,𝑖𝑛

𝑃𝐷𝐶= 1 −

1

𝐺

𝑃𝑅𝐹,𝑜𝑢𝑡𝑃𝐷𝐶

Power-Added Efficiency

• High Gain

• High Power

• DC Bias Point

• Low leakage


N-Polar GaN: Record 8.85 W/mm Pout at 94 GHz

Romanczyk et al., IEEE Trans. Electron Devices. Jan. 2018

0

2

4

6

8

Po

ut(W

/mm

)

Traditional Ga-Polar

N-Polar

Quaternary Ga-Polar

W-Band GaN Power Density

N-Polar breaks through Pout

saturation observed for traditional

Ga-polar devices with 8 W/mm

Outline


I. Introduction










IV. Conclusion

GaN HEMTs: Ga-Polar & N-Polar


Pad

Metal

Pad

Metal

Gate

AlGaN Backbarrier

GaN Buffer

SiC Substrate

MOCVD SiN

AlGaN Cap

GaN Channel

GaN Cap

2DEG

Regrown

n+Regrown

n+

Iso

latio

n

Iso

latio

n

Source

Ohmic

Drain

Ohmic

Ga-Polar HEMT Deep Recess N-Polar HEMT

-C

Planar N-Polar HEMT

Replace SiN Passivation

with GaN CapFlip Crystal Over

+C -C

Channel

Growth Direction

EC

EF

EV

Channel

EC

EF

EV

The N-Polar GaN Deep Recess HEMT Structure


N-Polar Deep Recess Structure

AlGaN backbarrier provides charge and 2deg confinement

Low resistance regrown n+ contacts by MBE

AlGaN cap & MOCVD SiN Gate Dielectric for low gate leakage

GaN Cap for dispersion control and low access resistance

Self-aligned gate for process control and low dispersionPad

Metal

Pad

Metal

Gate

AlGaN Backbarrier

GaN Buffer

SiC Substrate

MOCVD SiN

AlGaN Cap

GaN Channel

GaN Cap

2DEG

Regrown

n+Regrown

n+

Iso

latio

n

Iso

latio

n

Source

Ohmic

Drain

Ohmic Channel

Growth Direction

EC

EF

EV

Gate Buffer

GaN Cap Advantage #1: Access Region Conductivity


Access Region TLM

0

250

500

750

0 0.5 1

RS

H(Ω

/)

ID (A/mm)

Reference

Planar N-polar

Device

With GaN Cap

Channel conductivity improved over wide current range

Necessary for low Vknee

Polarization also reduces |E| in the

GaN channel improves mobility

mm-Wave Challenge: Controlling DC-RF Dispersion


n+ n+

AlGaN Cap

GaN Channel

AlGaN Backbarrier

2DEG

DS

G

Surface

States

Surface states exist in GaN devices

Charge state responds to DC bias

IDS

VDS

VG = 0V Static/DC

Vknee, Iknee



IDS

VDS

Vknee, Iknee

+VDS-VGS Ionized

Surface

States

n+ n+

AlGaN Cap

GaN Channel

DS

G

AlGaN Backbarrier

2DEG

Depleted Channel



VG = 0V Static/DC



IDS

VDS

VG = 0V Pulsed/RF

Vknee, Iknee

Pout and Drain Efficiency are degraded

relative to results expected from DC data

+VDS-VGS Ionized

Surface

States

n+ n+

AlGaN Cap

GaN Channel

DS

G

AlGaN Backbarrier

2DEG

Depleted Channel

𝑃𝑜𝑢𝑡 =𝑉𝐷𝐷 − 𝑽𝒌𝒏𝒆𝒆 𝑰𝒌𝒏𝒆𝒆

4

VG = 0V Static/DC



Traditional Solutions to Dispersion


SiN Passivation

SiN passivates surface states &

moves external surface far from

channel

Regrown

n+

Regrown

n+SiN Passivation

AlGaN Cap

GaN Channel

AlGaN Backbarrier

2DEG

DS

G

SiN + Field Plates

Regrown

n+

Regrown

n+SiN Passivation

AlGaN Cap

GaN Channel

AlGaN Backbarrier

2DEG

DS

G

Reduced electric field prevent trap

ionization

Capacitance penalty disallows

use at mm-wave frequencies

𝑉𝑇,𝑎𝑐𝑐𝑒𝑠𝑠 =𝑞𝑛𝑠𝐶

GaN Cap Advantage #2: Dispersion Control


Pad

Metal

Pad

Metal

Gate

AlGaN Backbarrier

GaN Buffer

SiC Substrate

MOCVD SiN

AlGaN Cap

GaN Channel

GaN Cap

2DEG

Regrown

n+Regrown

n+

Iso

latio

n

Iso

latio

n

Source

Ohmic

Drain

Ohmic

Planar N-Polar HEMT Deep Recess N-Polar HEMT

SiN passivation

replaced by GaN cap

0

10

20

30

40

50 150 250

VT

(V)

C (nF/cm2)

47.5 nm

GaN Cap

SiN

Increased ns outweighs differential in ϵr

Higher VT possible for same capacitance

47.5 nm GaN cap: VT,access ≈ 17 V

𝑉𝑇 =𝑞𝑛𝑠𝐶

Dispersion Control with N-Polar Deep Recess Structure


200 ns Pulsed I-V

Sub-10% dispersion

thru 16 V VDS,Q

Self-Aligned Deep Recess Fabrication Process


Process

MBE

Regrown n+

contacts

Implant

Isolation

GaN Cap

Recess Etch

MOCVD

SiN Gate

Dielectric

Gate

Metal

Ohmic

Metal

Pad Metal

1

2

3

4

5

6

7

Starting Epi:

Grown by MOCVD

(Haoran Li, Nirupam Hatui,

Athith Krishna)



Process

MBE

Regrown n+

contacts

Implant

Isolation

GaN Cap

Recess Etch

MOCVD

SiN Gate

Dielectric

Gate

Metal

Ohmic

Metal

Pad Metal

1

2

3

4

5

6

7

Starting Epi:

Grown by MOCVD


Athith Krishna)

GaN Etch:

GaN Cap Selectively Etched

BCl3/SF6 ICP Etch

AlGaN cap etch: BCl3/Cl2 RIE

SiO2

AlGaN Backbarrier

GaN Buffer

Substrate

AlGaN Cap

GaN Channel

GaN Cap

2DEG

MOCVD SiNALD Al2O3

Regrowth Hard Mask:

E-Beam Litho UV-N Process

ICP Etched SiO2 Pillars



Process

MBE

Regrown n+

contacts

Implant

Isolation

GaN Cap

Recess Etch

MOCVD

SiN Gate

Dielectric

Gate

Metal

Ohmic

Metal

Pad Metal

1

2

3

4

5

6

7

MBE n+ Regrowth:

(Elaheh Ahmadi, Karine

Hestroffer, Christian Wurm)

Plasma Assisted MBE

20 nm UID + 30 nm n+

NSi ~ 1020 cm-3

Rsh: 85 to 125 Ω/sq.



Process

MBE

Regrown n+

contacts

Implant

Isolation

GaN Cap

Recess Etch

MOCVD

SiN Gate

Dielectric

Gate

Metal

Ohmic

Metal

Pad Metal

1

2

3

4

5

6

7

Implant Isolation(Teledyne Scientific & Imaging)

MBE n+ Regrowth:

(Elaheh Ahmadi, Karine

Hestroffer, Christian Wurm)

Plasma Assisted MBE

20 nm UID + 30 nm n+

NSi ~ 1020 cm-3

Rsh: 85 to 125 Ω/sq.



Process

MBE

Regrown n+

contacts

Implant

Isolation

GaN Cap

Recess Etch

MOCVD

SiN Gate

Dielectric

Gate

Metal

Ohmic

Metal

Pad Metal

1

2

3

4

5

6

7

Gate Recess Hard Mask

Deposition

SiO2 Patterning

E-Beam Litho CSAR Mask

ICP SiO2 Etch

GaN Cap Selective Etch

BCl3/SF6 ICP Etch



Process

MBE

Regrown n+

contacts

Implant

Isolation

GaN Cap

Recess Etch

MOCVD

SiN Gate

Dielectric

Gate

Metal

Ohmic

Metal

Pad Metal

1

2

3

4

5

6

7

MOCVD SiN Gate Dielectric


Anchal Agarwal, Athith Krishna)



Process

MBE

Regrown n+

contacts

Implant

Isolation

GaN Cap

Recess Etch

MOCVD

SiN Gate

Dielectric

Gate

Metal

Ohmic

Metal

Pad Metal

1

2

3

4

5

6

7

Gate Formation

EBL UV6 Pattering defines the

top gate

SiO2 Defines the gate stem

Gate Metal: Cr/Au (45/500nm)

Gate Release

SiO2 Hard Mask etched

MOCVD SiN Gate Dielectric


Anchal Agarwal, Athith Krishna)



Process

MBE

Regrown n+

contacts

Implant

Isolation

GaN Cap

Recess Etch

MOCVD

SiN Gate

Dielectric

Gate

Metal

Ohmic

Metal

Pad Metal

1

2

3

4

5

6

7

False Color SEM

30° tilt

Gate Release

SiO2 Hard Mask etched



Process

MBE

Regrown n+

contacts

Implant

Isolation

GaN Cap

Recess Etch

MOCVD

SiN Gate

Dielectric

Gate

Metal

Ohmic

Metal

Pad Metal

1

2

3

4

5

6

7

Ohmic Metal Ti/Au (20/100 nm)

Pad Metal Ti/Au/Ni (30/650/30 nm)

Regrown

n+

Regrown

n+

Outline


I. Introduction










IV. Conclusion

N-Polar GaN Deep Recess Device Overview


WG = 2×37.5 µm tchannel: 12 nm

LG = 75 nm LGS = 75 nm LGD = 250 nm

Physical Parameters

460 mS/mm ID = 1.8 A/mm

Ron = 0.4 Ω-mm

Off-State Breakdown: 38 V

Regrown

n+

Regrown

n+

Small Signal Gain


113 GHz peak fT 238 GHz peak fmax

Large Signal Device Evaluation: 94GHz Load Pull


7.94 W/mm max Pout at

20V (26.9% PAE)

16V VDS,Q

500 mA/mm (~25% IDS,max)20V VDS,Q

500 mA/mm (~25% IDS,max)

Romanczyk et al. IEEE Trans. Electron Devices. Jan. 2018

28.8% Peak PAE at 16V

(5.3 W/mm Pout)

Comparison with Literature


𝑃𝑜𝑢𝑡 =𝑉𝑠𝑤𝑖𝑛𝑔 × 𝐼𝑠𝑤𝑖𝑛𝑔

8≤

2 × 𝑉𝐷𝑆,𝑄 × 𝐼𝑚𝑎𝑥

8


N-Polar GaN: Record W-Band Performance

N-Polar offers greater current

density giving higher Pout

Record-high combination

of PAE and Power Density

Closed Symbols: N-Polar (This Work) Open Symbols: Ga-Polar GaN

Constant 8 W/mm: 10 – 94 GHz


First Demonstration of Constant Pout from 10 – 94 GHz

(as expected from ideal FET operation)

94 GHz: UCSB Passive Load pull

30 GHz: Maury Microwave MT2000

Active Load pull

10 GHz: UCSB Passive Load pull

Romanczyk et al. IEEE Trans. Electron Devices. Jan. 2018

500 mA/mm IDS,Q


Ka-Band Performance (30GHz)

At 30 GHz increased gain allows access to deeper Class AB operation

500 mA/mm 220 mA/mm

~25% IDS,max ~11% IDS,max

15.2 dB Glinear 13.8 dB Glinear

55.9% PAE 59.8% PAE

5.6 W/mm 4.9 W/mm

Romanczyk et al, GOMACTech 2018

Ka-Band Load Pull of GaN Devices


N-Polar offers greater

current density

Record-high combination

of PAE and Power Density

Romanczyk et al, GOMACTech 2018

Two-Tone Linearity at 30 GHz


Pin

(d

Bm

)

f (GHz)P

ou

t (d

Bm

)

f (GHz)

fo

f2

f1 = fo – Δff2 = fo + Δf2Δf ~ 10 MHz

fo

f1

f2f1

Input

Linear Output

Pou

t (d

Bm

)

Pin (dBm)

OIP3

IIP3

1:13:1

Pf1

PI3

Pou

t (d

Bm

)

f (GHz)fo

2f2-f12f1-f2+ additional higher

order terms

f2f1

Distorted Output

IM3highIM3lo

This Work

DARPA DREAM Program BAA

DARPA DREAM Program Goal

Two-Tone Linearity at 30 GHz


10V VD, 300 mA/mm

• PI3 slope near 3:1

• 35 dBm OIP3

• 11dB OIP3/PDC

Figure of Merit

M. Guidry et al, EuMW 2019

Pou

t (d

Bm

)

Pin (dBm)

OIP3

IIP3

1:13:1

Pf1

PI3

Summary


• Frequency-Independent Pout

• 94GHz: Record 8 W/mm Pout: 4x improvement over traditional Ga-Polar GaN HEMTs

28.8% Peak PAE

• 30GHz: Record High GaN PAE: 59.8%

Record high combinations of PAE and Pout

11 dB OIP3/PDC

Large-Signal Performance

• Inverted polarization fields enable the Deep Recess HEMT design

Enhanced Access Region Conductivity

Control of DC-RF Dispersion

N-polar Deep Recess HEMT Advantage

Documents

N-Polar GaN Deep Recess HEMTs for mm-Wave Power …